Numerous circuit devices, such as transistors, diodes and resistors, have operating characteristics that are temperature dependent. To accurately test a device to determine whether it is operating within prescribed limits, its temperature during the test procedure should accordingly be known. Because of its temperature dependence the device may itself be used as a temperature sensor, in which case it is especially important to determine whether it is operating accurately.
Since the operating characteristics of various devices are temperature dependent, it is at least conceptually possible to determine the device temperature by exciting an input signal to the device, observing a temperature-dependent output signal, and calculating the temperature from the relationship between the two signals. For example, germanium and silicon diodes have been operated at a constant forward-biased current and the resulting forward-biased voltage has been measured to determine the temperature in accordance with the standard forward-bias diode equation [1]:ΔV=T·k/q·ln(I1/I0)  [1]wherein I0 is a small forward energizing level, I1, is an integer multiple (typically 10) of I0, ‘ln’ is the natural logarithm function; q is the elementary charge (1.602 176 462(63)×10−19 Coulomb); k is the Boltzmann constant (1.380 6503 (24)×10−23 J K−1); T is the absolute temperature in Kelvin and ΔV is the voltage difference between V1, and V0, respective responses of I1, and I0 energizing levels.
Conventional devices according to U.S. Pat. No. 6,554,469 of Thomson et al. and U.S. Pat. No. 6,554,470 of Zhang et al. are quite restrictive in the choice of the group of current excitation levels. U.S. Pat. No. 6,554,469 demands an integer ratio between 4 current levels. In U.S. Pat. No. 6,554,470 the current levels are limited to 3 degrees of freedom. Four current level ratios are demanded for the best performance of their methods. Such a need adds complexity in the design of the current conditioner, making their specified ratios more driftable with aging, increase ohmic temperature cycling of the PN-junction sensor and put it in a higher electrical stress that will accelerate aging. At U.S. Pat. No. 6,554,469 one can read “the calculations necessitated by a three-current approach are likely to require non-integer math, which can be difficult and/or impractical to implement”. This difficulty will be overcome by the embodiments of this application.
The method disclosed by Thomson et al. in U.S. Pat. No. 6,554,469 is a four-current version of a prior work made within Analog Devices known from U.S. Pat. No. 5,195,827 of Audy et al. As stated by their authors, these methods are better suited for bipolar transistors than for diodes.
Zhang discusses in U.S. Pat. No. 6,554,470 the inability of Audy's method to take in account parallel parasitic effects. Zhang shows the deviations in temperature measurement for current levels above 2 mA, compared to Audy's method, however, using Zhang's I/V data, one will find that this method is less accurate than conventional devices and furthermore, if one goes above 1 mA, unnecessary concerns of self-heating will be raised.
Olfa Kanoun offered a compact behavioural model for PN-junction temperature measurement in her PhD. thesis “Neuartige Modelle zur kalibrationsfreien Temperaturmessung mit pn-Übergängen”, Fortschrittberichte, Reihe 8, Nr. 905, VDI-Verlag, 2001. This model is described in two conference papers, “IEEE Instrumentation and Measurement Technology Conference Budapest, Hungaria May 21-23, 2001 and Conference Sensors and Systems, Jun. 24-27, 2002, Saint-Petersburg, Russia. This model is sensitive, at least in a simulation, to diode serial resistances above the kilo-Ohm level.
Alekseevich et al. claims in the document RU 2,089,863 an enhanced sensitivity and accuracy for temperature measurement using PN-junctions, with alternating forward and backward currents. This known circuitry requires a 3-wire sensor device, which is a drawback in respect to provide a simple and effective device for measuring temperatures by PN-junction.
From document WO 0,023,776 of Eryurek et al. The auto correction of aging effects for RTD temperature sensors is known. These sensors still need to be calibrated at the factory. To achieve the same temperature sensitivity and using the same thin film lithography fabrication technology, RTDs are huge when compared to a PN-junction or a series of PN-junctions. This embodiment requires a four-wire connection to a single sensor, which is even more complicated and costintensive than the known solution of Alekseevich et al. implying a 3-wire sensor device.
From Lunghofer et al. U.S. Pat. No. 5,713,668 the use of a self-verifying thermal sensor is known using 2 thermocouples and one RTD and the issue of thermal coupling the 3 sensors.
Dukor et al. presents in SU 1,527,515 and SU 1,543,250 two arrays of diodes as thermal sensors, but they have not presented an accurate physical model for PN-junction temperature measurement to make use of such an embodiment as an auto-calibrated and redundant sensor.